Film Condensation inside Horizontal Tubes

Our discussion of film condensation so far has been limited to exterior surfaces, where the vapor and liquid condensate flows are not restricted by some overall flow-channel dimensions. Condensation inside tubes is of considerable practical interest because of applications to condensers in refrigeration and air-conditioning systems, but unfortunately these phenomena are quite complicated and not amenable to a simple analytical treatment. The overall flow rate of vapor strongly influences the heat-transfer rate in the forced convection-condensation system, and this in turn is influenced by the rate of liquid accumulation on the walls. Because of the complicated flow phenomena involved we shall present only two empirical relations for heat transfer and refer the reader t.o Rohsenow [37] for more complete information. 

Chato [38] obtained the following expression for condensation of refrigerants at low vapor velocities inside horizontal tubes  

This equation is restricted to low vapor Reynolds numbers such that 

The mass velocities for the liquid (7, and vapor (7, are calculated as if each occupied the entire flow area. Equation (9-31) correlates experimental data within about 50 percent when 

A vertical square plate, 30 by 30 cm, is exposed to steam at atmospheric pressure. The plate temperature is 98°C. Calculate the heat transfer and the mass of steam condensed per hour. 

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Heat Transfer Correlations for Film Condensation 581 10-6 Film Condensation Inside Horizontal Tubes 591 10-7 Dropwise Condensation 591... 

For film condensation inside horizontal tube, the Chato (1962) correlation is recommended for low vapor inlet Reynolds number (<35,000)... 

Select the calculation method to be used. Condensation inside horizontal tubes can be predicted assuming two mechanisms. The first assumes stratified flow, with laminar film condensation. The... 

Inside Horizontal Tubes Condensation inside horizontal tubes can be predicted assuming two mechanisms laminar film condensation and vapor shear dominated condensation in which the two-phase flow is in the annular region. For laminar film condensation the further assumption is made that the rate of condensation on the stratified layer of liquid running along the bottom of the tube is negligible. Consequently, this layer of liquid must not exceed values assumed without being approximately accounted for. 

For low vapor velocities, film condensation heat transfer inside horizontal tubes can be determined from... 

Condensation may also take place inside horizontal tubes, the flow regimes depend strongly on the velocity of the vapor. At low vapor flows, the condensate film tends to collect... 

In genera], the coefficient of a film condensing on a horizontal tube is considerably larger than that on a vertical tube under otherwise similar conditions unless the tubes are very short or there are very many horizontal tubes in each stack. Vertical tubes are preferred when the condensate must be appreciably subcooled below its condensation temperature. Mixtures of vapors and noncondensing gases are usually cooled and condensed inside vertical tubes, so that the inert gas is continually swept away from the heat-transfer surface by the incoming stream,... 

C-K Chen and S-A Yang, Laminar Film Condensation Inside a Horizontal Elliptical Tube with Variable Wall Temperature, Int. J. Heat and Fluid Flow, 15, pp. 75-78,1994. 

Stagnant saturated steam at 100°C condenses on the outside of a horizontal copper tube with outside diameter 5.0 cm. The copper tube has a wall thickness of 1.5 mm and a thermal conductivity of 390 W/m-K. At the axial location being considered, the bulk temperature of the vater flowing inside the copper tube is 80°C and the inside heat transfer coefficient is 3500 W/m2. Assuming film condensation, calculate the heat transfer and condensation rate per unit length of pipe. 

So far we have discussed film condensation on the outer surfaces of tubes and other geometries, which is characterized by negligible vapor velocity and the unrestricted flow of the condensate. Most condensation processes encountered in refrigeration and air-conditioning applications, however, involve condensation on the inner surfaces of horizontal or vertical tubes. Heat transfer analysis of condensation inside tubes is complicated by the fact that it is strongly influenced by the vapor velocity and the rate of liquid accumulation on the walls of the tubes Q ig. 10-34). 

Cox et al. [101] used several kinds of enhanced tubes to improve the performance of horizontal-tube multiple-effect plants for saline water conversion. Overall heat transfer coefficients (forced convection condensation inside and spray-film evaporation outside) were reported for tubes internally enhanced with circumferential V grooves (35 percent maximum increase in U) and protuberances produced by spiral indenting from the outside (4 percent increase). No increases were obtained with a knurled surface. Prince [102] obtained a 200 percent increase in U with internal circumferential ribs however, the outside (spray-film evaporation) was also enhanced. Luu and Bergles [15] reported data for enhanced condensation of R-113 in tubes with helical repeated-rib internal roughness. Average coefficients were increased 80 percent above smooth-tube values. Coefficients with deep spirally fluted tubes (envelope diameter basis) were increased by 50 percent. 

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